1. Field of the Invention
The present invention is directed to an X-ray diagnostic installation of the type having a storage luminescent screen for the latent storage of an X-ray image, an X-ray exposure unit for generating an X-ray beam, and a readout system, which causes the storage luminescent screen to luminesce by being scanned with a radiation source, a detector for the acquisition of the light emitted by the storage luminescent screen, and an image playback system.
2. Description of the Prior Art
German OS 38 03 766 discloses an X-ray diagnostic installation of the above type, wherein a storage luminophore is irradiated with X-rays as a radiation-sensitive transducer, so that unbonded electrons, that are stored in traps, are generated therein. The entire surface of this storage luminescent screen is scanned pixel-by-pixel by an additional radiation source of a readout system, for example a laser, so that the electrons stored in the traps are excited and can fall back into recombination centers, with the energy difference being emitted in the form of light quanta. As a result, it is possible to readout the stored X-ray image from the storage luminescent screen.
For planar scanning of the storage luminescent screen, a laser beam of the type referred to as a xe2x80x9cflying spot scannedxe2x80x9d is deflected by two mirrors in the vertical and horizontal directions, so that all picture elements lying on the storage luminescent screen are successively scanned. The light emitted by the storage luminescent screen is acquired by two light-guiding plates and is conducted onto two laterally attached, line-shaped CCD light transducers. The output signal of the detector is supplied to a normal video chain for playback of the X-ray image on a monitor.
Europium-activated barium fluoride-bromine chloride compounds disclosed in German OS 33 47 207 that can be excited by visible light (photo-stimulation) can be employed as storage luminophores. A He-Ne laser that generates light at a wavelength of 633 nm is usually for the stimulation of this storage luminophore.
European Application 859 244 discloses an X-ray diagnostic installation wherein the plates of the storage luminescent screens are stimulated region-by-region by a laser with preceding fiber optics instead of by the xe2x80x9cflying spot scannerxe2x80x9d. The emitted light of the irradiated surface is acquired by matrix of CCD image converters which are likewise preceded by fiber optics. For complete scanning of the storage luminescent screen, the readout system is moved over its entire surface.
In such X-ray detectors according to the storage luminophore principle, an X-ray projection image of the patient is first produced and is read out by the scanner and digitized. Such detectors allow only large-format radiographic exposures with a relatively high dose to be made, and these must be slowly read out to obtain full resolution. Subsequent excerpt enlargements with correspondingly adapted resolution (zoom) are not available with such detectors.
An object of the present invention is to provide an X-ray diagnostic installation of the type initially described that enables a fast synoptic exposure and a subsequent zoom exposure with high resolution.
This object is inventively achieved in an X-ray diagnostic installation operable in a first operating mode wherein the X-ray exposure unit produces X-ray images with a first dose and the readout system implements a fast readout of the image area of the storage luminescent screen with low resolution, and in a second operating mode, wherein the X-ray exposure unit produces X-ray images in a limited region with a higher dose compared to the first dose, and this part of the image area of the storage luminescent screen is read out with high resolution. As a result, a synoptic exposure can be produced first, an image excerpt, and thus a region to be scanned, is then selected, and a high-resolution detail enlargement is then made with electronic zoom.
It has proven advantageous for the scanning (readout) radiation source to be line-shaped and for the detector to be line-shaped and switchable in terms of its resolution.
Inventively, the focusing range of the line-shaped radiation source can be switchable, for example between two focus settings. Given a switchable radiation source, the line width of the CCD is not crucial, so that the line-shaped detector need be switchable between two resolutions only in the line direction.
Optimum adaptations of the installation can be achieved in an embodiment wherein the line-shaped radiation source has two separate arrangements for generating stimulation light at different focus settings and wherein the line-shaped detector has two optically and electrically separate CCD lines with different resolutions.
If a switchable radiation source is not used, then, in the second operating mode, a number of photo-elements can be repeatedly read out simultaneously in the direction of the line width and in chronological succession, their signals being added pixel-by-pixel.
Advantageously, the resolution in the second operating mode can amount to nine times the resolution in the first operating mode, and the dimensions of the active photo-elements for a picture element in the second operating mode can amount to 50xcexcm and the dimensions of the active photo-elements for a picture element in the first operating mode can amount to 150 xcexcm.